A tool used to perform a surgical procedure is the bur. A bur generally consists of a head formed from rigid material, typically metal, shaped to have a number of flutes. The flutes are formed to define tissue cutting edges. A shaft extends rearwardly from the head. The free end of the shaft has a feature that facilitates locking the shaft to a powered handpiece. The actuation of the handpiece results in the rotation of the bur. During a surgical procedure, the bur head is placed against a surgical site where a section of tissue is to be removed. The rotating cutting edges excise tissue away from the surgical site. Burs of various shapes and sizes are used in procedures such as orthopedic surgery, neuro and spinal surgery, ear noise and throat surgery and in other surgical procedures in which a sub-procedure is to selectively remove a section of tissue.
Burs work well for the purposes for which they are designed. Nevertheless, a problem associated with some burs is chatter. Chatter is the back and forth vibration of a bur head against the surface to which the bur head is applied. Chatter occurs as a result of bur's individual cutting edges repeatedly being forced against the tissue against which the bur head is applied. Generally, there three reasons a bur may start to chatter.
One reason a bur starts to chatter is because it receives an input of energy due to a process known as regeneration of waviness. This process is due to the fact that when a cutting edge passes across a section of tissue, it leaves a specific wavy (essentially sinusoidal) profile along the surface of the tissue. If two adjacent cutting edges cut in phase, the second cutting edge excises tissue along a surface profile identical to that along which in was excised by the first flute. In practice, due to the invariable movements of the bur head and the tissue, this does not happen. When any two successive cutting edges pass over the same tissue section, the second flute cutting edge removes tissue on a path that does not overlap the tissue wave excised by the first cutting edge. Consequently, the debris chips cut by the second cutting edge have variable thickness. This means, during the process in which the second cutting edge excises the chip from the tissue, the cutting edge and its flute are subjected to variable forces. Over time, the repetitive exposure of the bur flutes to these variable forces causes the bur to undergo forced vibration.
A second reason a bur may chatter is that it is rotated at its resonant frequency. If this occurs, the repetitive force against the flutes self-excites the bur to move back and forth through a continually increasing range of motion.
The third reason a bur may chatter is due to the depth of the cut in the tissue against which the bur head is applied. If a bur head is pressed against the tissue so as to make only a relatively shallow cut, the overall time any two adjacent flutes are exposed to the tissue being cut is relatively low. The time in which the two adjacent flutes, as well as the spatial gap between the flutes, are exposed to the open environment is relatively high. During these relatively long time periods, tissue cut from bur and entrained in this gap is able to be discharged away from the bur head. This gap is then relatively debris-free the next time it rotates against the in-place tissue. Additional newly excised tissue fills this gap. However, if the bur head is pressed against the tissue to make a deep cut, the time in which the spatial gap between any two flutes is located against the in-place tissue increases. The time this gap is exposed to the open environment drops. Consequently, there may not be enough time for tissue entrained in this gap to be discharged. These gaps between the flutes clog. If this occurs, during the next time period in which the gap is rotated against the in-place. It is believed that that build up of chips between the flutes and their cutting edges clogs the bur head. This clogging, in turn, it is believed causes a forced vibration and the resultant chatter.